1 Deep inside the Earth's minerals lies a little bit of geometry. Every mineral has a specific crystalline structure. This geometric structure is the result of the careful arrangement of the minerals' atoms, ions, or molecules. It is this specialized geometric structure that helps scientists to categorize the Earth's minerals.2 Crystals have an internal structure that contains regular repeating patterns. Although some minerals may have large crystals, many have crystals that can only be seen under a microscope. It is the larger mineral crystals that scientists study to understand the internal geometric structure of crystals. As minerals grow, there may be certain conditions that restrict the growth of large crystals. This is why some minerals form masses of very small crystals. Minerals will form single large crystals when their growth is not affected by outside conditions. These silicate mineral crystals will take on one of six shapes: isolated tetrahedra, ring silicates, single-chain silicates, double-chain silicates, sheet silicates, and framework silicates. Scientists view the crystal shapes of minerals through x-rays. The x-rays pass through the crystal and strike photographic plates. Once this occurs, an image of the crystal structure is produced on the plate. This is how scientists know the geometric patterns of atoms in the crystalline structures.3 The crystalline structure of silicate minerals contains four oxygen atoms that are arranged in pyramids with a silicon atom in the center. Scientists call this arrangement a silicon-oxygen tetrahedra. For every different type of silicate minerals, there are different combinations of silicon-oxygen tetrahedrons. Each arrangement is different based on the bonds between the oxygen atoms and the other atoms in the tetrahedra. Scientists have noticed that tetrahedra bonds may form between the oxygen and silicon atoms and other neighboring tetrahedra. There may also be bonds between the oxygen atoms and atoms from other elements, which are outside of the tetrahedra. Now let's explore the six types of tetrahedra.